Pilot control valves for use in controlling the application of a control fluid to a controlled valve are old in the art. There are numerous variations of valving element configurations designed to be responsive to pressures, springs or combinations thereof. Also old in the art are variations in valving element pressure areas (i.e., stepped pressure areas) for controlling the pilot valve in a certain manner in response to changes in actuating pressure (pilot pressure). Some pilot valve include external valving element handles for manually shifting the element to control the fluid flow therethrough. Additionally, certain pilot valves of this nature have included a means for locking the pilot valve in the in-service position, independent of pilot pressure, when such pilot pressure is below a predetermined amount. A previous application, Ser. No. 922,066, filed July 5, 1978 and now abandoned, and Snyder U.S. Pat. No. 4,355,658 to the same subject matter, describe one possible design.
However, existing locking apparatus designs exhibit certain deficiencies. For instance, pilot valves are frequently used in the petroleum, petrochemical, or other industries to control the passage of flammable or explosive fluids. In the event of an emergency, such as a fire, the lockout apparatus would remain engaged, thereby inadvertently preventing or alternatively allowing fluid to continue to flow, possibly creating or contributing to potentially severe consequences.
Thus, it becomes desirable to provide a pilot valve locking apparatus that may be disengaged manually, but also automatically disengages in response to certain changes either in pilot fluid pressure or to an elevated temperature as would be experienced in case of a fire.